At the time of cargo handling work such as lifting and moving heavy goods, an industrial vehicle such as a forklift is used. As the industrial vehicle, an industrial vehicle in which an arm for a cargo handling operation or power steering for steering assistance is operated by oil pressure is widely used. This industrial vehicle is provided with a hydraulic pump that is driven by an engine, and configured such that hydraulic oil discharged from the hydraulic pump is supplied to a hydraulically-operated device that is operated by oil pressure.
However, in recent years in which improvement in fuel consumption or regulation of emission gas has been carried out from viewpoints of global environment protection or the like, an engine of minimum performance according to an output that is required has been used. In a case of using such an engine, if the load of a hydraulic pump increases, the torque of the engine is insufficient. Thus, a hydraulic pump cannot be driven, and a phenomenon in which rotation of the engine stops (a so-called engine stall) occurs. Then, since such an engine stall leads to a sudden stopping of operation of an industrial vehicle, reliable preventive measures are desired.
Therefore, an industrial vehicle provided with a hydraulic system which can prevent occurrence of such an engine stall is proposed in the related art (refer to, for example, PTL 1). FIG. 5 is a schematic diagram showing a hydraulic system of an industrial vehicle 70 related to an example of the related art. The industrial vehicle 70 includes a pump for cargo handling 72a, a pump for steering 72b, a control valve 73, hydraulic piping 74a and 74b, discharge piping 75, an unloading valve 76, a rotation speed sensor 77, a pressure sensor 78, and a control device 79.
The pump for cargo handling 72a and the pump for steering 72b are driven by an engine 71 so as to discharge hydraulic oil. The control valve 73 controls supply of the hydraulic oil to a hydraulically-operated device for a cargo handling operation (not shown) and a hydraulically-operated device for steering (not shown). The hydraulic piping 74a and the hydraulic piping 74b connect the pump for cargo handling 72a and the pump for steering 72b to the control valve 73. The discharge piping 75 branches from the hydraulic piping 74a and is connected to a tank. The unloading valve 76 selectively opens or closes the discharge piping 75. The rotation speed sensor 77 detects the rotation speed of the engine 71. The pressure sensor 78 detects the pressure of the hydraulic oil flowing through the hydraulic piping 74a. The control device 79 controls an operation of the unloading valve 76 on the basis of input signals from the rotation speed sensor 77 and the pressure sensor 78.
According to such a hydraulic system, at the time of usual cargo handling work, a state is created where the discharge piping 75 is closed by the unloading valve 76, and thus all of the hydraulic oil discharged from the pump for cargo handling 72a is sent to the control valve 73 through the hydraulic piping 74a. 
Further, all of the hydraulic oil discharged from the pump for steering 72b is sent to the control valve 73 through the hydraulic piping 74b and is shunted into the inside of the control valve 73, thereby being preferentially supplied to the hydraulically-operated device for steering. Then, surplus hydraulic oil which has not been supplied to the hydraulically-operated device for steering is supplied to the hydraulically-operated device for a cargo handling operation.
On the other hand, if the rotation speed of the engine 71 becomes less than or equal to a predetermined value, or if the pressure of the hydraulic oil becomes greater than or equal to a predetermined value, the control device 79 determines that there is a possibility that an engine stall may occur, and controls an operation of the unloading valve 76, thereby opening the discharge piping 75. Then, all of the hydraulic oil discharged from the pump for cargo handling 72a is discharged from the hydraulic piping 74a through the discharge piping 75 to the tank. In this way, since the load of the pump for cargo handling 72a is reduced, occurrence of an engine stall due to a shortage of the torque of the engine 71 can be prevented before it happens.